Root canal and periapical pathosis develops upon introduction of bacteria or bacterial products into the root canal space. Root canal infections are polymicrobial and consist of approximately 90% anaerobic organisms. The root canal space of a necrotic pulp acts as a closed environment as it lacks collateral circulation. The hypothesis of my research is changes in microbial communities in root canals toward a more virulent state occur as a result of specific bacterial interactions and physicochemical changes in that environment. Specific aims include 1) examining potential interactions between endodontic pathogens and other oral bacteria in the development of mixed microbial communities and 2) determine the effects of physicochemical changes in the microenvironment on the development of microbial communities and expression of virulence determinants. Findings suggest that 1) stimulatory and/or inhibitory factors may be produced by specific bacteria in mixed batch culture affecting the development of a community of organisms, 2) oxygen protective factors of blood are related to the red blood cells and the lysis of the RBC stimulates the growth yield of P. endodontalis in an anaerobic environment, and 3) addition of blood alters the growth and development of microbial communities associated with endodontic pathosis. Future studies are to examine the mechanism of iron acquisition from RBI's and the potential role of hemolysin activity.